Photocatalytic film of iron oxide, electrode with such a photocatalytic film, method of producing such films, photoelectrochemical cell with the electrode and photoelectrochemical system with the cell, for the cleavage of water into hydrogen and oxygen

ABSTRACT

The Photocatalytic film of semiconducting iron oxide (Fe 2 O 3 ), contains an n-dopant, or a mixture of n-dopants, or a p-dopant or a mixture of p-dopants. Electrode consists of a substrate, with one or more films or photocatalytic arrangements of film of semiconducting n-doped or p-doped iron oxide (Fe 2 O 3 ) e.g. on the surface of one side of the substrate or on the surface of different sides. The photoelectrochemical cell comprises electrodes with a film or with films of the n-doped or p-doped semiconducting iron oxide (Fe 2 O 3 ). The semiconducting iron oxide (Fe 2 O 3 ) film can be manufactured with a spray pyrolysis process or a sol gel process. The system for the direct cleavage of water with visible light, into hydrogen and oxygen the system comprises one or more of the photoelectrochemical cells with photocatalytic films. The system can be a tandem cell system, comprising the photoelectrochemical cell with the doped iron oxide (Fe 2 O 3 ) film.

FIELD OF THE INVENTION

[0001] The present invention relates to a photocatalytic film of ironoxide according to the preamble of claim 1. The invention relatesfurther to an electrode with such a film, to a method of producing sucha film and to the precaution of an electrode composed of iron oxide(Fe₂O₃) which upon illumination with light oxidizes water to oxygen withhigh quantum efficiency and a system with such an electrode in a tandemcell for water cleavage by visible light or sunlight.

[0002] It is known that iron(III) oxide is a semiconductor that producesoxygen from water tinder illumination. The reaction involves positivecharge carriers (holes) generated in the valence band of the oxide byphoto-excitation with light whose energy exceeds the band gap energy ofthe semiconductor. In contrast to most oxides the band gap of iron oxideis sufficiently small, i.e. 2.2 electron volts, to allowphoto-excitation by visible light. This renders it attractive for use asa photo-catalyst in a system affording the cleavage of water bysunlight. However the photo-oxidation of water is notoriouslyinefficient on Fe₂O₃ unless single crystal electrodes are employed,whose cost is prohibitively high for practical applications.

[0003] The task of the present invention is to provide an improvedphotocatalytic film and an electrode with such a film or such films.Further it is an object of the invention to provide for a method forproducing an improved polycrystalline film of iron oxide that e.g.accomplishes tie photo-oxidation of water in an efficient manner.Another object of the invention is an improved system for the cleavageof water. The film can be used in a tandem cell accomplishing thecomplete cleavage of water into hydrogen and oxygen by sunlight.

[0004] The photocatalytic film of iron oxide is characterized by thefeatures of the characterizing part of claim 1. The electrode accordingto the invention shows the features of the characterizing part of claim5. The method of producing the film is characterized by the features ofthe characterizing part of claim 8. And the system for cleavage of waterwith visible light contains one or more of the photoelectrochemicalcells according to the invention.

DESCRIPTION OF THE INVENTION

[0005] The invention and in particular the photoelectrochemicalproperties of the film and the electrode are illustrated by way ofexample by tie accompanying diagrams.

[0006]FIG. 1 shows the current-voltage (I-V) characteristics of Ti⁴+doped iron oxide films or layers of various doping levels.

[0007]FIG. 2 shows the current-voltage (I-V) characteristics of Sb⁵+doped iron oxide films or layers of various doping levels.

[0008]FIG. 3 shows the photocurrent action spectra of iron oxide dopedwith the different dopants, that is Sb. Ti compared with iron oxide thatis not doped. IPCE %, means (Incident Photon to Current Efficiency inpercents). The diagram in the inset shows the absorption spectra of ironoxide as a function of the wavelength of the light.

[0009]FIG. 4 illustrates the photocurrent action spectra of films withdifferent numbers of superimposed iron oxide layers. The diagram in theinserted window shows the absorbance of iron oxide films with differentnumber of layers (thickness) for light of different wavelength.

[0010]FIG. 5 illustrates the I-V characteristics of iron oxide in thepresence of sodium lactate as a sacrificial donor. The supportingelectrolyte is aqueous NaCLO₄ at pH=1.06.

[0011]FIG. 6 shows a schematic drawing of a so-called tandem cell thatoperates as a water photolysis device.

[0012] A method is disclosed for the preparation of an electrodeconstituted by a compact polycrystalline Fe₂O₃ film supported on aconducting glass. The latter serves as a transparent electric currentcollector. The film is an n-type semiconductor, electrons being themajority carriers and holes the minority charge carriers or a p-typesemiconductor. The n-doping is achieved by introducing a suitablesubstitutional dopant into the Fe₂O₃ lattice. When placed in contactwith the electrolyte a depletion layer is formed at the semiconductorsurface. The electric field present in the space charge layer assists inthe separation of electron-hole pairs produced by light excitation ofthe oxide. This effect is critical in order to obtain high quantumyields for photocatalytic oxygen evolution on Fe₂O₃. Other factors thatcontrol the efficiency of the Fe₂O₃-films are the minority carrierdiffusion length determined by impurities or lattice imperfections andthe presence of surface states acting as recombination centers. Thelatter factors are very unfavorable for conventional Fe₂O₃ filmsimpairing its operation as an anode for photocatalytic water oxidation.The method disclosed in the present invention overcomes these materialproblems.

[0013] Spray pyrolysis is a process where a solution containing theconstituents of the film to be produced is e.g. sprayed onto a heated orhot substrate. The solvent evaporates or boils of and the film isproduced by pyrolysis of the remaining constituents contained in thesolvent.

[0014] An alternative for the production of films is the sol gelprocess.

WORKING EXAMPLE

[0015] Preparation of Fe₂O₃ films on conducting glass andphotoelectrochemical characterization: A spray pyrolysis procedure wasemployed to prepare Fe₂O, layers. An alcoholic (2-propanol) solutioncontaining 0.1M FeCl₃.6H₂O and 0.1 M HCl was sprayed on to a uniformlyheated (380° C.) conducting glass open to an air atmosphere. The spraygun was operated using nitrogen as a carrier cum atomizing gas.Substitutional doping of other elements such as Ti, Sb, Si, etc. wereintroduced into the Fe₂O₃ film by premixing suitable precursors(chlorides, for example) in the FeCl₃ solution employed for the spraypyrolysis. These dopants are able to augment the number of the majoritycarriers (electrons). The spray time was fixed at 10 seconds, each time.The time interval between two consecutive spraying acts was 10 minutes.Multiple spraying was used to increase the layer thickness and producingan arrangement of films consisting of two or more layers of the film.

[0016] A three electrode (working, counter and reference) setup was usedfor evaluating the electrochemical performance of the finishedelectrodes. Upon contacting the Fe₂O₃film with the aqueous electrolyte,a depletion layer is formed whose electric field assists in theseparation of electron-hole pairs generated by light. This driveselectrons to the bulk and then to the external circuit and the holes tothe Fe₂O₃ surface where they oxidize water to oxygen. Concomitantly atthe Pt counter electrode, or counter-electrode made of anotherconducting material, H₂ is liberated as a result of the reductionreaction. Alternative materials to platinum for use as acounter-electrode are for example nickel or polytungstate derivativessupported on a conductive substrate. In FIGS. 1 and 2, the anodicphotocurrent is plotted as a function of the electrode potential forvarious Fe₂O₃ electrodes. The effect of the dopant is to augmentstrongly the photocurrent at a given potential. An almost 4 foldincrease of the photocurrent is obtained, under reverse bias. Within therange of concentrations employed, Ti⁴⁺ doping showed the bestperformance at 5 atom % (FIG. 1). With Sb⁵⁺, this was observed at 0.125atom % (FIG. 2). Both the onset potential of the photocurrent and itsmaximum value were taken for such comparison. By contrast, substitutionof iron by silicon which is most frequently used to n-dope iron oxidedid not improve the performance of the films.

[0017] The spectral response of the photocurrent is shown in FIG. 3.Between the two dopants, it was found that Ti⁴⁺ doping shifts thespectral response of the layer more towards visible wavelength (redshift), when compared to Sb⁵⁺ dopant. Sb doping also helps in spectralred shift but less effectively. Such spectral shift will improve thelight harvesting property of the electrode.

[0018] The effect of number of sprayed layers was also studied. Resultsare shown in FIG. 4. It was observed that even though the lightabsorbance increases with increasing number of layers, the highest IPCE(incident photons to current efficiency) was noticed with 7 layers.Thereafter, the IPCE started decreasing again.

[0019] The effect of adding sacrificial donors to the electrolytesurrounding Fe₂O₃ electrode was also investigated. FIG. 5 shows theimprovement in the current voltage characteristics of such a system.

[0020] The Photocatalytic film of semiconducting iron oxide (Fe₂O₃),contains an n-dopant, or a mixture of n-dopants, or a p-dopant or amixture of p-dopants. Electrode consists of a substrate, with one ormore films or photocatalytic arrangements of film of semiconductingn-doped or p-doped iron oxide (Fe₂O₃) e.g. on the surface of one side ofthe substrate or on the surface of different sides. Thephotoelectrochemical cell comprises electrodes with a film or with filmsof the n-doped or p-doped semiconducting iron oxide (Fe₂O₃). Thesemiconducting iron oxide (Fe₂O₃) film can be manufactured with a spraypyrolysis process or a sol gel process. The system for the directcleavage of water with visible light, into hydrogen and oxygen comprisesone or more of the photoelectrochemical cells with photocatalytic films.The system can be a tandem cell system, comprising thephotoelectrochemical cell with the doped iron oxide (Fe₂O₃) film.

[0021] A schematic drawing of a water photolysis device, a tandem cell,is illustrated in FIG. 6. The device consists of two photo systemsconnected in series. The cell on the left contains the aqueouselectrolyte that is subjected to water photolysis. The electrolyte iscomposed of water as a solvent to which an electrolyte has been addedfor ionic conduction. Saline seawater can also be used as a water sourcein which case the addition of electrolyte becomes superfluous. Lightenters from the left side of the cell through a glass window 1. Aftertraversing the electrolyte 2 it impinges on the back wall of the cellconstituted by a mesoporous semiconductor film 3 composed of dopedFe₂O₃. The latter is deposited onto a transparent conducting oxide film4, made from a material such as fluorine doped tin dioxide that servesas current collector which is deposited on the glass sheet 1. The oxideabsorbs the blue and green part of the solar spectrum while the yellowand red light is transmitted through it. The yellow and red part of thesolar spectrum is captured by a second cell mounted behind the back wallof the first cell. The second cell contains a dye sensitized mesoporousTiO₂ film. Its functions is a light driven electric bias increasing theelectrochemical potential of the electrons that emerge-from the filmunder illumination to render the reduction of water to hydrogenpossible. It consists of a transparent conducting oxide film 4 depositedon the back side of the glass sheet 1 constituting the back wall of thefirst cell. The conducting oxide film is covered by the dye-derivatizednanocrystalline titania film 6. The latter is in contact with theorganic redox electrolyte 7 and the counter electrode 8 consisting of aglass which is rendered conductive on the side of the organicelectrolyte by deposition of a transparent conductive oxide layer.Behind the counterelectrode there is a second compartment 9 containingan aqueous electrolyte of the same composition as in the frontcompartment 2. Hydrogen is evolved at the cathode 10 which is immersedin the second electrolyte compartment. The two electrolyte compartments2 and 10 have the same composition and are separated by an ionconducting membrane or a glass frit 11.

[0022] We shall now discuss a specific embodiment of such a tandemdevice achieving the direct cleavage of water into hydrogen and oxygenby visible light. A thin film of nanocrystalline tungsten trioxideabsorbs the blue part of the solar spectrum.

WO₃ +hν

WO₃(e−, h ⁺)

[0023] The valence band holes (h⁺) created by band gap excitation of theoxide serve to oxidize water forming oxygen and protons:

4h ⁺+H₂O

O₂+4H⁺

[0024] while the conduction band electrons are collected on theconducting glass support forming the back wall of the first photocell.From there on they are fed into the second photocell that consists of adye sensitized nanocrystalline TiO₂ film. The latter is mounted directlybehind the WO₃ film capturing the green and red part of the solarspectrum that is transmitted through the top electrode. The role of thesecond photocell is merely that of a photo driven bias. Theelectrochemical potential of the electrons is sufficiently increased bypassing through the second photocell that they can reduce the protonsproduced during water oxidation to hydrogen.

4H⁺+4e ⁻

2H₂

[0025] The overall reaction corresponds to the splitting of water byvisible light.

H₂O

H₂0.5O₂

[0026] Semiconducting oxides, such as WO₃ and Fe₂O₃ are the materials ofchoice for the photo-anode as they are stable under operation resistingto both dark and photo corrosion. Tungsten trioxide and ferric oxide areso far the only known and readily available oxide semiconductors thatare capable of producing oxygen using visible light. The electronsgenerated in the oxide are collected by the conducting glass and aresubsequently fed into a second photocell that is placed directly behindthe oxide film. The photo-active element of this second cell is a dyesensitized mesoporous TiO₂ capturing the yellow and red light that istransmitted through the oxide electrode. It serves as a photo-drivenbias increasing the electrochemical potential of the photoelectronsproduced by band gap excitation of the oxide to render reduction ofwater to hydrogen feasible.

1. Photocatalytic film of semiconducting iron oxide (Fe₂O₃), characterized in that it contains an n-dopant, or a mix of n-dopants, or a p-dopant or a mix of p-dopants.
 2. Photocatalytic film according to claim 1, the n-dopant or the n-dopants selected from the elements Si, Ge, Pb, Ti, Zr, Hf, Sb, Bi, V, Nb, Ta, Mo, Tc, Re, F, Cl, Br, I, Sn, Pb, N, P, As or C or the p-dopant or the p-dopants selected from the elements Ca, Be, Mg, Sr, Ba,
 3. Photocatalytic film as claimed in claim 1 of 2, the film having a thickness between 100 nm and 10,000 nm, preferably the film having a thickness between 500 nm and 5000 nm.
 4. Photocatalytic arrangement of films, consisting of two or more layers of film as claimed in any of claims 1 to
 3. 5. Electrode characterized by a substrate, on the substrate one ore more films or photocatalytic arrangements of film of semiconducting iron oxide (Fe₂O₃) as claimed in any of claims 1 to
 4. 6. Electrode according to claim 5, the substrate and the one or more films being transparent for light with a wavelength of 600 nm or more.
 7. Electrode according to claim 5 or claim 6, wherein the substrate is a metal, crystal, glass, an electrically conducting glass, or plastic.
 8. Method of producing a polycrystalline photocatalytic film of Fe₂O₃ or arrangement layers of films according of any of claims 1 to 4, or of producing an electrode according to any of claims 5 to 7, characterized in that the layer or layers are produced by a spray pyrolysis process or a sol gel process.
 9. Photoelectrochemical cell with one or more electrodes as claimed in any of claims 5 to 7, a polycrystalline photocatalytic Fe₂O₃ film or arrangement of layers of films, as claimed in any of claims 5 to
 7. 10. System for the direct cleavage of water with visible light, into hydrogen and oxygen the system comprising one or more of the photoelectrochemical cell as claimed in claim 9
 11. System for the direct cleavage of water with visible light, into hydrogen and oxygen as claimed in claim 10 comprising a tandem cell consisting of two superimposed photoelectrochemical cells, both cells being electrically connected, the photoactive material present in the top cell being a film or an arrangement of layers of films of Fe₂O₃ deposited on a conducting glass and placed in contact with an aqueous electrolyte solution, said oxide absorbing the blue and green part of the solar emission spectrum to generate oxygen and protons from water and transmitting the yellow and red light to a second photoelectrochemical cell, mounted, in the direction of the light, behind the top cell and comprising of a dye sensitized photovoltaic film, said bottom cell converting the yellow, red and near infrared portion of the light in particular sunlight, to drive the reduction of the protons, produced in the top cell during the water oxidation process, said reduction of protons to hydrogen gas taking place in an electrolyte compartment mounted behind the bottom cell and being separated from the top cell compartment where oxygen is evolved by a glass frit or an ion conducting membrane. 